Microwave circuit carriers come in a variety of forms, including microwave chip carriers (MCC's) and monolithic microwave integrated circuit (MMIC) packages. Typically, MCC's and MMIC packages are designed to house one semiconductor die and associated impedance matching devices such as capacitors and resistors. Subsystem carriers, however, which accept a variety of device packages, typically house multiple dies. Existing single or multiple microwave device carriers are generally hermetically sealed to conform with Military environmental standards. However, commercial demand for higher speed communication has fostered the increased use of faster switching semiconductor dies such as Gallium Arsenide (GaAs) for non-military applications. Carriers using GaAs chips are generally expensive assemblies requiring complex fabrication methods and contain numerous and various piece parts.
In addition, known carriers have difficulty in translating the performance of high frequency microwave chips to a system level due to the multiple transmission line impedance transitions required as the result of multiple materials and the type of materials incorporated in current carrier designs. To optimize signal transmission (or reception) from inside the carrier (from the MMIC die) to eventual propagation mediums such as waveguides or coaxial lines, line impedance is primarily controlled through line geometry changes and additional components (separate coaxial connectors mounted to the carrier are also used).
Traditionally, additional and separate (not integral to the carrier) probe pins or coaxial connectors (and cables) have provided the final transition between the GaAs die and the propagation medium. These microwave coupling devices add cost, increase the system complexity, and increase transmission line losses (energy radiated from transitions resulting from the use of these components). Unfortunately, no microwave semiconductor die package, known to the inventors, contains an integral propagation medium coupler, formed from the same material comprising the package, that is capable of mechanically and electrically coupling directly to a waveguide.
Moreover, as the size of microwave integrated circuits increases (integration of more circuit stages) and components are added to a substrate's surface, component density increases; therefore, varying line width becomes a limiting factor in highly dense carrier designs. Varying the thickness of the dielectric also varies the impedance of a transmission line, but existing substrates must generally undergo an additional etching or cutting process, thereby adding cost and increasing fabrication complexity.
Energy loss due to unmatched transmission lines is a critical parameter in designing high frequency microwave carriers, particularly when signal frequencies of 12 gigahertz or higher are being coupled from the die to the carrier and then to a printed circuit. Limited frequencies of operation result from the failure to minimize the number and types of transitions to effectuate proper interconnection to an external system. Known surface mount carriers aid in limiting transmission line lengths (reduce signal propagation delays) between the die and a subsequent propagation medium, but typically only house single MMIC's, have limited pinouts and have hermetic seals (expensive for many commercial or consumer applications).
Piece parts normally associated with microwave device carriers include: a separate coaxial connector to interface to an external medium, a ceramic planar dielectric substrate to host the die, hybrid circuitry and any necessary impedance matching components, a machined metal cavity housing to limit electromagnetic interference (EMI) and radio frequency interference (RFI) (it also provides structure for the hermetic seal), and a lead frame to interconnect the dies to an external printed circuit board.
Ceramic is generally used as the primary substrate material due to its dielectric properties, thermal expansion properties, and ability to host thin film (or thick film) hybrid circuitry. These properties aid in reducing transmission line losses and allow line widths to be readily varied. Ceramic's inability to be readily molded for customized designs and its high cost serve as barriers in realizing a lower cost commercial microwave device package. Not only is the ceramic substrate expensive, but the machined metal cavity and the metal coaxial connector also add weight and cost, and further restrict the size and outline of the carrier (or package) beyond acceptable limits. The problem of multiple and various types of transmission line transitions still exists with this type of package.
Hermetically sealed, ceramic flat pack surface mount packages, as understood in the art, have addressed the problems of bulk and cost associated with the machined cavity by eliminating it. Instead, energy-shielding cavities are formed by via holes, filled with conductive material such as conductive epoxy or a co-fired tungsten paste, that project through the substrate and act as conductive walls at microwave frequencies. These vias are grounded, forming a grounded cavity about the MMIC chip. Another type of shield is the seal ring which is not tied to ground and has a specified seal ring resonance. Although this package incorporates different resonant cavity designs, and is surface mountable, it still comprises a planar ceramic substrate, lead frame and other piece parts, and requires multiple and various types of transmission line transitions (including through wall transitions) to couple to a waveguide or coaxial line.
Existing subsystem carriers attempt to offer economical interconnection methods primarily between MMIC's, while providing a production-oriented design; but not between MMIC's and propagation mediums such as waveguides. One such package uses a grid array resembling a waffle. This package contains a series of dielectric coated wires running between recessed receiving locations for packaged or unpackaged chips.
This system requires layers of metal foils between wires to minimize cross-coupling and also requires manufacturing intensive wire routing to the appropriate devices. However, coupling to a waveguide must be done through an additional transition piece (probe pins). Although these subsystems may provide acceptable interconnection between multiple MMIC's, they require employ of a separate waveguide coupler and are relatively high in cost due partially to their fabrication complexity, and therefore find limited use in low cost commercial applications.
From the foregoing discussion, it is evident that existing microwave device carriers suffer in economy and performance due to the type of substrate used (primarily ceramic), the types of transitions needed, and the number of piece parts needed to form a hermetic package. Recent developments in polymer technology have resulted in plastic substrates as circuit carriers for sub-microwave frequency applications. Molded "three-dimensional" printed circuits are known only for low frequency applications. Printed circuits are considered "three-dimensional" when the substrate comprises a surface variation in at least one dimension.
These molded circuit carriers are plastic and formed by an injection molding process. Therefore, they may be shaped to avoid or adapt to physical constraints in a specific application. Patterns of conductors are metallized to predetermined surfaces of the substrate and serve as circuit traces and bonding pads for circuit components. Once components are affixed, it becomes a multi-device carrier. Plastic is generally a lossy and non-hermetic material and has not traditionally found use as substrate material in microwave carrier designs.
Since current impedance-matching techniques in gallium arsenide carriers and packages vary transmission line widths and thicknesses and add external components to control unmatched impedances, providing an inexpensive variable thickness substrate would provide an additional impedance-controlling mechanism not traditionally found in microwave frequency applications. Readily varying the thickness of the substrate on a per-conductor basis allows the carrier designer greater flexibility when designing dense MMIC carriers.
Accordingly, there exists a need for a surface mount, high frequency microwave circuit carrier and integral waveguide coupler, having a three-dimensional shape wherein its shape may be molded to accommodate specific applications. It should reduce traditional piece part counts to effectuate a reduction in cost and energy losses and provide an insulating member capable of additional impedance varying capabilities on a per conductor basis. It should also be capable of accepting more than one microwave device. In addition, interconnections between multiple devices should be achieved by low-cost manufacturing techniques.